This invention relates to a fluorescent lamp for unipolar mode of operation such as is described in "Lichttechnik", 30th Year, No. 3, 1978, pages 106 to 108.
Such a fluorescent lamp is filled predominantly with mercury vapour, while the inner face of the glass envelope thereof is coated with a fluorescent material. Unipolar mode of operation is a power supply with direct current or with a clocked d.c. voltage, namely with a voltage pulsating with only one polarity, such as a sinusoidal half-wave voltage, a rectangular pulse train etc. A direct current operation, especially of low pressure gas discharge lamps, is such that for physically plausible and experimentally proved reasons--so far as ohmic stabilizing resistances can be avoided--the same leads to a light yield which, as compared to the conventional alternating current operation, is improved by 20% and more.
Yet in the direct current operation mercury ions migrate from the anode to the cathode, which is why the anode region undergoes a mercury impoverishment, whereby the light yield in the anode region is decreased. This phenomenon is also called cataphoresis.
Now in order to reduce cataphoresis, in the case of the fluorescent lamp mentioned in the first paragraphs hereinabove and constructed as a double tube lamp it has been proposed to use a special diaphragm which is permeable to mercury vapor yet which is gas-discharge-tight. However, the use of a suchlike diaphragm and the construction of a double tube lamp represent a considerable expenditure.
From "Technical Newsletter", December 1984, Vol. 6, No. 6, pages 1 and 2 is known a fluorescent lamp the cathode of which is permanently heated in order to provide ideal conditions for igniting the arc discharge and for maintaining the same, as well as a sufficiently high Hg vapour pressure which, in the case of a high switching frequency, is advantageous.